In control of a synchronous machine having permanent magnets as a field system using a synchronous machine control device having power conversion means such as an inverter, a phenomenon referred to as “demagnetization” is known to occur whereby the strength of the magnetization of the permanent magnets of the field system, i.e. the magnetic flux, decreases with rises in temperature, for instance on account of energization of the armature windings of the synchronous machine, or due to iron loss in the synchronous machine itself. A further phenomenon known to occur is “irreversible demagnetization” in which once an allowable temperature is exceeded, the magnetic flux cannot return to the state prior to the occurrence of demagnetization, even if the temperature drops to normal temperature.
When controlling a synchronous machine having permanent magnets as a field system, it becomes therefore necessary to at least control the temperature of the permanent magnets not to be higher than an allowable temperature at which irreversible demagnetization occurs. It is deemed that demagnetization also results in drops in torque. Meanwhile, it is difficult to directly attach temperature detectors to permanent magnets, for instance due to space-related problems in the structure of the synchronous machine, or because of the surroundings thereof being protected using a case.
Many synchronous motors having permanent magnets as a field system often have the magnets inside the rotor. Therefore, such a configuration constitutes yet another serious obstacle in terms of attaching temperature detectors.
Accordingly, technologies are demanded that allow measuring or estimating indirectly, by resorting to some method, the temperature of permanent magnets, or magnetic flux correlated to the temperature of permanent magnets, to allow keeping the temperature of permanent magnets at or below an allowable temperature.
In some conventional devices (for instance, PTL 1), as examples of such synchronous machine control devices aimed at solving the above problems, the below-described steps are sequentially carried out so as to determine the demagnetization state of a rotor magnet portion.
Step ST1: measurement of rotational speed and current-voltage.
Step ST3: estimation of winding temperature on the basis of the above measured values of rotational speed and current-voltage.
Step ST4: estimation of winding resistance on the basis of the estimated value of winding temperature.
Step ST5: estimation of the temperature of the rotor magnet portion on the basis of the estimated value of winding temperature.
Step ST6: estimation of a normal value of induced voltage on the basis of the estimated value of winding temperature.
Step ST7: estimation of the actual value of induced voltage on the basis of the estimated value of the winding resistance.
Step ST8: comparing the normal value of the induced voltage coefficient with the actual value, as estimated in step ST6 and step ST7, and determining that demagnetization has occurred when the result obtained exceeds a predetermined threshold value.
Other examples of similar control devices include conventional devices (for instance, PTL 2) in which a control scheme such as the below-described one is carried out so as to estimate the temperature of permanent magnets on the basis of a sum of revolutions, iron loss and mechanical loss.                Detecting the revolutions of the rotor rotating in a non-energized state through disconnection from a load.        Estimating the magnet temperature of the rotor on the basis of the detected revolutions.        Working out a correction amount for correcting a current command for the synchronous machine (permanent magnet electric motor), on the basis of the estimated magnet temperature, and driving the synchronous machine on the basis of the correction amount.        